The present invention is generally directed to an RF power amplifier system for use in amplifying an RF input signal and is more particularly directed toward checking and adjusting the value of quiescent currents flowing through transistors employed in the system.
RF power amplifier systems are known in the art for use in amplifying RF signals for broadcasting purposes, including radio and television. Such a power amplifier system is disclosed in our U.S. Pat. No. 6,188,277 assigned to the same Assignee as the present application. This patent is hereby incorporated herein by reference. Such a power amplifier system employs a plurality of sub-amplifiers operating together in parallel with each receiving and amplifying a portion of an RF input signal. The amplified portions are then combined to provide a combined output signal. Each sub-amplifier includes at least one transistor having an input port that receives a DC bias voltage and an output port. When the RF input signal is not present, a quiescent current flows through the output port. The transistors may take the form of MOSFET transistors.
All LDMOS transistors have a characteristic called bias or Idq drift. Early versions of these devices had significant drift, which created a problem for designers. Some improvements in device design and processing have resulted in lower drift parts becoming available. Unfortunately, drift has not gone away completely. All CMOS devices are susceptible to degradation of performance caused by a phenomenon known as hot carrier injection. This is a consequence of the combination of the oxide layer used to insulate the active elements in these devices in combination with the strong electric fields resulting from small device geometries. Hot carrier injections occurs when electrons with higher than average energy (the xe2x80x9chotxe2x80x9d electrons) are accelerated into the dielectric layer (the injection) resulting in microscopic damage to the oxide layer. The xe2x80x9ctrapsxe2x80x9d created by this damage collect charge over time.
MOSFET transistors are CMOS devices designed for high frequency, high voltage operation, and all parts exhibit the hot carrier injection effect to some degree. Hot carrier injection results in charge build-up in the gate-drain region, which causes the gate field to change. The user sees this as a change in the quiescent current (Idq) with a fixed gate voltage. The projected drift of Idq is usually from 10 to 25% over 20 years of the device life span. Idq drift affects the level of distortion for the RF signal generated by the device. According to device manufacturer""s data there is an optimum level of Idq that allows minimum amount of distortions. Deviation from it in either direction will degrade the intermodulation (IMD) performance of the device. Initially aligned for optimum performance the device will eventually de-tune and will require additional alignment.
The most common solution is a short burn-in of the device in a circuit prior to final alignment of the current. As a consequence of the logarithmic time relationship, approximately 50% of the Idq drift that will occur in a projected 20-year operating lifetime occurs in the first 24 to 36 hours of operation. A relatively short burn-in, especially if the drift is accelerated, will reduce the bias drift during the equipment operating life. Acceleration is accomplished by operating the device at an elevated Vds without RF drive. Burn-in process increases the final cost of the product without completing the task. The Idq is still going to change, however at a much slower rate.
The temperature of a power amplifier affects the magnitude of the quiescent current. The magnitude of this quiescent or bias current changes over a period of time as the device ages. At this time, it is not practical to adjust the quiescent current once the product is in the field for the reasons as discussed above. Preferably, a controller should be provided that automatically checks and adjusts the magnitude of the quiescent current flowing in each of the transistors. This is provided by the invention herein.
In accordance with one aspect of the present invention, a power amplifier system is provided for use in amplifying an RF input signal. The system includes N sub-power amplifiers connected together in parallel with each receiving and amplifying a portion of the RF input signal. A combiner combines the amplified portions of the RF input signal and provides a combined output signal. Each sub-amplifier includes at least one transistor having an input port that receives a DC bias voltage and an output port, such that when the RF input signal is not present, a quiescent current flows through the output port. A controller is operative, when the RF signal is not present, to check and adjust the magnitude of the quiescent current flowing in each of said sub-amplifier output ports in the sequence of 1 . . . N.